The present invention relates to acoustic printing, and more particularly to controlling the on/off switching ratio between ejectors of an acoustic printhead.
The fundamentals of acoustically ejecting droplets from an ejector device such as a printhead has been widely described, and the present assignee has obtained patents on numerous concepts related to this subject matter. In acoustic printing, an array of ejectors forming a printhead is covered by a pool of liquid. Each ejector can direct a beam of sound energy against a free surface of the liquid. The impinging acoustic beam exerts radiation pressure against the surface of the liquid. When the radiation pressure is sufficiently high, individual droplets of liquid are ejected from the pool surface to impact upon a medium, such as paper, to complete the printing process. The ejectors may be arranged in a matrix or array of rows and columns, where the rows stretch across the width of the recording medium, and the columns of ejectors are approximately perpendicular.
Ideally, each ejector when activated ejects a droplet identical in size to the droplets of all the other ejectors in the array. Thus, each ejector should operate under identical conditions.
In acoustic printing, the general practice is to address individual ejectors by applying a common RF pulse to a segment of a row, and to control the current flow to each ejector using column switches. In some cases it is desirable to use one column switch for several rows in parallel in order to reduce the number of column driver chips and wire bonds, and hence cost, in the system. Unfortunately, this approach results in parasitic current paths which can cause undesired RF current to flow through ejectors that are not in an ON state.
In existing systems, the switching ratio is limited and will vary with the number of ejectors that are ON in a given row. A switching ratio is defined as the RF power in an OFF ejector to the RF power in an ON ejector (i.e. POFF/PON).
FIG. 1 illustrates an acoustic switching array with a desired current path for a selected row and selected column for an existing system. Switching matrix 10 is a 4-row 12a, 12b, 12c, 12d by 64 column 14a, 14b, 14zz switching matrix. Rows are connected to the matrix via switching elements 16a, 16b, 16c, 16d, and columns are connected through switching elements 18a, 18b, 18zz. At the intersection of the columns and rows are transducers 20. Current paths of matrix 10 are terminated at RF ground 21. It is to be appreciated that while the matrix of FIG. 1 is a 4-row by 64-column matrix, the present invention may be used in other matrix designs.
Matrix 10 is supplied by a power source 22 which provides its output to an RF signal matching circuit or controller 24. By proper switch sequencing, a desired current path for a selected row and selected column is obtained. For example in FIG. 1, by closing switch 16a and switch 18a, a current path is provided from RF matching network 24 to a transducer 20a via row 12a and column 14a. As the remaining rows and columns are unselected, only transducer 20a is intended to be activated to emit a droplet.
Unfortunately, the interconnect paths used to implement a low cost acoustic printhead include unavoidable, undesirable current paths, as shown and discussed for example, in connection with FIGS. 2-4.
FIG. 2 is a simplified depiction of an undesired current path through an unselected transducer in the same row as a selected transducer. In this example, switches 16a and 18a are maintained in a closed position while the remaining switches are unselected. Therefore current is provided to transducer 20a. However, undesired current will also flow through transducer 20b, which is in selected row 12a but unselected column 14b. Similarly, FIG. 3 illustrates a situation where an undesired current flows through transducer 20c, which is in selected column 14a and unselected row 12c. 
Column switches 18a-18zz are, in one embodiment, implemented with a component such as a PIN diode, which has a reasonably high intrinsic switching ratio, i.e., in the range of xe2x88x926 dB or greater. A high switching ratio of this type may insure that a particular column switch is securely turned OFF if it were the only device in the system. However, a net switching ratio of a selected column and a selected row ejector (relative to other ejectors which should be OFF) can vary between approximately xe2x88x922.3 dB and xe2x88x926 dB, depending upon the number of existing parasitic current paths through ejectors which are not selected.
Turning to FIG. 4, a more detailed discussion is provided regarding the parasitic current paths introduced in connection with FIGS. 2 and 3. The transducers are identified by the row and column numbers to which they are connected. For the case illustrated, all current paths start from the conductor of row0, 12a, and terminate at RF ground return 21.
In the following example, transducer 20b is an unselected transducer. The undesired current through unselected transducer 20b consists of three components, all of which start from row0, 12a, and proceed down through transducer 20b. The first component flows from transducer 20b, down through the top segment of column1, 14b, up through transducer 20d, through a segment of row1, 12b, down through transducer 20e, down through column0, 14a, and finally through the selected column0 switch, 18a, to RF ground return 21.
The path of the second component is from row0, 12a, down through transducer 20b, and the top two segments of column1, 14b, up through transducer 20f, through a segment of row2, 12c, down through transducer 20c, down through column0, 14a, and finally through column0 switch, 18a, to RF ground return 21.
The path of the third component is from row0, 12a, down through transducer 20b, and the top three segments of column1, 14b, up through transducer 20g, through a segment of row3, 12d, down through transducer 20h, down through column0, 14a, and finally through column0 switch 18a, to RF ground return 21.
It is to be noted that no significant current is assumed to flow through any of the open (unselected) switches in columns 1 through 63 (14b-14zz), and rows 1, 2 or 3 (12b-12d).
Unwanted current paths, similar to those just described, also exist through other unselected transducers located on row0, 12a, and columns 2 through 63 (14b-14zz).
Transducers 20e, 20c, and 20h have the largest magnitude of total unwanted current. For example, the current flowing through the unselected transducer 20e is the sum of the currents in all the other transducers in row1, 12b. All of this unwanted current flows through the conducting path of unselected row1, 12b. In this example, transducers 20e, 20c and 20h are on a selected column and unselected rows. The switching ratio is the poorest for this category when only one column is selected. This may also be seen in FIGS. 6 and 7.
In the following description it is to be noted that FIGS. 5, 6, 9, 10, 15, 16 and 18 are block diagrams representing four categories of transducer states used in calculations of relative RF currents to determine the switching ratios for different numbers of selected columns.
For example, in FIG. 5 the block in the upper left part of the figure represents all of the transducers that are at Selected Row, Selected Column locations. Similarly, the block in the upper right part of the figure represents all of the transducers that are at Selected Row, Unselected Column locations. The block in the lower left part of the figure represents all of the transducers that are at Unselected Row, Selected column locations, and the block in the lower right part of the figure represents all of the transducers that are at Unselected Row, Unselected Column locations.
Turning more particularly to FIGS. 5 and 6, set forth are similar simplified depictions of switching matrix 10. FIG. 5 illustrates a situation where 63 columns 26, and one row 12a are selected, ON, and a single column 28 and remaining three rows 12b-12d are unselected, or OFF. Under this arrangement, the inventors have calculated that there is approximately 514 xcexcA flowing through each of the 63 transducers 30, which represents the transducers in selected row 12a, and 63 ON columns 26 of matrix 10. It was also determined by this analysis that 393 xcexcA of current will flow in transducer 32, located in selected row 12a and the 64th unselected column 28 of transducers. With this information, it is found that the switching ratio between these two currents is equal to:
393 xcexcA/514 xcexcA=0.765=xe2x88x922.32 dB.
FIG. 6 depicts an alternative arrangement where one column 34, and one row 12a are selected, and remaining 63 columns 36 and 3 rows 12b-12d are unselected. In this situation, the selected current path for transducer 38 has a current of 504 xcexcA, whereas an unwanted current of approximately 368 xcexcA exists through each of the unselected transducers connected to selected column 34 and unselected rows 12b-12d. This results in a switching ratio equal to:
368 xcexcA/504 xcexcA=0.730=xe2x88x922.73 dB.
The cumulative current through switch 18a is approximately 1607 xcexcA (i.e. 504 xcexcA from the selected transducer in column 34, row 12a and 368 xcexcA from each of the three unselected transducers on column 34, on rows 12b-12d).
FIG. 7 summarizes the effective switching ratios relative to selected row/unselected columns, and selected columns/unselected rows as a function of the number of ejectors in the row which are ON. Particularly, curve 40 shows that as the number of selected columns increase, for an unselected row, the relative switching ratio improves substantially, i.e. approximately to xe2x88x9210 dB at 20 columns selected. Alternatively, curve 42 illustrates that for a selected row, as additional columns are moved to an ON state, the switching ratio is degraded substantially, i.e. from about xe2x88x9211 dB at one column ON, to about xe2x88x922.5 dB for 63 columns ON.
When using aqueous inks for acoustic ink printing, the desired ejection velocity will be approximately 4 m/sec. This can be achieved using approximately 1 dB of power over the ejection threshold. Given that there are ejection threshold power non-uniformities in the aqueous printhead of approximately +/xe2x88x920.5 dB, and the desire to maintain some margin of safety (e.g. xe2x88x920.5 dB) to insure that ejectors which are unselected are truly OFF, an appropriate switching ratio may be found by the restrictions of: switching ratio (SR) less than (overdrive for 4 m/sec)xe2x88x92(non-uniformity)xe2x88x92(margin to insure appropriate OFF state), which results in:
SRxe2x89xa6(xe2x88x921xe2x88x920.5xe2x88x920.5)=xe2x88x922 dB.
Therefore, a switching ratio of xe2x88x922.5 to xe2x88x923.0 dB will be acceptable for printing of aqueous inks, when a xe2x88x920.5 to xe2x88x921.0 dB safety margin is added.
However, and more specifically related to the present invention, phase-change inks require more power over the threshold than aqueous inks. To achieve a necessary 4 m/sec ejection velocity, it has been determined that a xe2x88x924 dB power over the threshold will be required. For phase-change inks, it is intended to use static E-fields to reduce this power requirement, however it is still necessary to eject the droplets at approximately 2 m/sec, i.e. xe2x88x922 dB over threshold. Non-uniformities in the phase-change printhead can be larger than in aqueous printheads (i.e. +/xe2x88x921 dB), and the margin for turning the switches fully OFF will also be similar (i.e. xe2x88x920.5 dB). Therefore, the switching ratio for phase-change inks will require:
SRxe2x89xa6(xe2x88x922 xe2x88x921 xe2x88x920.5)=xe2x88x923.5 dB.
Then, with a 0.5 to 1.0 dB safety margin added, a switching ratio of xe2x88x924.0 to xe2x88x924.5 dB is acceptable. Existing switching networks do not insure adequate switching ratio for phase-change printing when the foregoing requirements are taken into consideration.
A further complication which exists for phase-change printing, is that thermal uniformity requirements are more exacting than for aqueous printing because the acoustic losses in the ink are larger, and phase-change inks change more strongly with temperature than aqueous inks. As a result, a several degrees celsius change across the printhead, or between the ON and OFF states of a given ejector can result in spatial and time-varying non-uniformities which will degrade output. For example, a 1-2xc2x0 C. change can result in a degradation in the drop diameter uniformity of 1-3%. It is believed the upper limit on drop diameter non-uniformity that can be tolerated for acceptable print quality is only 5%. Thus even comparatively small changes in temperature will cause printing degradation.
The foregoing problem is particularly acute for low flow printheads, i.e. printheads where the ink is not quickly passed through the printhead. In these situations, the acoustic energy will raise the temperature of the focal region above the bulk of the ink. In some printheads the temperature rise has been determined to be as much as 12xc2x0 C. While this temperature rise can be used to an advantage (i.e. reducing the temperature requirement for the bulk volume of the ink in the printhead), it poses a problem of non-equal thermal environments for ejectors that are ON versus those that are OFF.
It has, therefore, been determined desirable to provide thermal environments for droplet ejector ON and OFF states which are essentially identical, by obtaining a specific switching ratio which balances the thermal changes associated with a hot ejected ink droplet. It is also considered beneficial to provide a controlled, specific switching ratio which is independent of the number of ejectors of any array which are ON and OFF.
Thus, it would be desirable to increase the switching ratio, and provide means to control the switching ratio at a desired level, independent of the number of ejectors which are ON.
Provided is a compensation circuit which can inject additional current into, or remove current from, a switching mechanism of a printhead to control the switching ratio. The additional compensation network allows for the maintaining of a precise switching ratio, independent of the number of ejectors in an ON state, in order to limit thermal non-uniformities in a printhead. The compensation design allows for a controlled adjustment of the switching ratio, and allows for control of the switching ratio at a desired level independent of the number of non-ejectors.
In a more limited aspect of the present invention, the compensation network is designed to take advantage of the design features of an acoustic ink printhead. In particular, since the configuration of the acoustic ink printhead results in up to 50% of heat energy at a focal spot being removed by ejection of a droplet, additional energy can be supplied such that a temperature balance is maintained between ejectors in an ON state and those in an OFF state.